Some track type and skid steer machines use a hydrostatic transmission for propelling the machines. The hydrostatic transmissions have a pair of overcenter variable displacement pumps connected to a pair of hydraulic drive motors through closed loop circuits. Forward travel is achieved by simultaneously controlling the displacement of the variable displacement pumps to direct fluid to the motors in one direction while reverse travel is achieved by simultaneously controlling the displacements of the pumps to direct fluid to the motors in the opposite direction. Steering is accomplished in both forward and reverse travel by reducing the output of one pump while maintaining the output of the other pump at the preselected forward or reverse travel setting.
Heretofore, the output of the pumps for both the forward and reverse travel and the steering functions have been controlled by four pressure reducing valves and four ball resolvers with the pressure reducing valves being actuated by a joystick mechanism. However, that type of system produces an unnatural "S" shape turn when a directional change is made during a steering maneuver. Another problem encountered with that system is that when the engine of the machine lugs during a turning maneuver, the machine tends to straighten from the desired turn path. Another problem is that most pump controls interact with the joystick pressure reducing causing noticeable instability. Finally, most hydrostatic pilot controls do not provide counter rotation turns or spot turns in neutral.
Thus, it would be desirable to provide a control system for a hydrostatic transmission in which the machine travels over the same path when a directional change is made during a steering maneuver and in which a demanded turn does not straighten if the engine lugs during a turning maneuver.
The present invention is directed to overcoming one or more of the problems as set forth above.